If I play the piano for extended periods(2 hours+) I get terrible back pain.
What is the correct distance/height for the digital piano/chair and what should my posture look like?
Without knowing all of the dimensions - your height, leg length, arm length, upper to lower body proportion, height of existing chairs, size of books, etc. etc., it's almost impossible to answer sensibly. However, what you use is patently obviously not good!
Everyone will sit in a slightly different posture to play. Ideally, the keyboard starting height needs to be about the height of a standard piano keyboard. That then puts you in a start position, and is about the only 'standard' dimension. Arms are more important than legs: elbows slightly higher than wrists. I disagree with @mramosch that only fingers play. Arms and even shoulders come into the equation to add weight, when needed. Where your legs end up will be wherever, as the height of the stool/chair will dictate that. It may cause slight problems with pedaling, but that's to get used to. Distance between keys and pedals is fixed on proper pianos. You may well be slouching as you play. That's not good for pain whatever you are doing. Vertical back is good.
The pain is God's way of saying you're not doing it right, so it's good that you address this issue. As ever, a teacher would solve the problem toute suite,, but if it continues, play for less time, and/or consult a doctor. It may be that there is another problem that has been revealed in your piano playing.
Let your arms and hands hang down to the left and right of your torso. Note the posture of your relaxed fingers. Now lift your arms and put your fingers in exactly that posture on the keyboard. That is the perfect position of your fingers.
As for your arms: the part from your fingers upwards should be horizontal up to your elbows. Never let your body put any pressure onto your arms and fingers. Only your fingers play. Just stretch your elbows a little bit outwards away from your torso so they don't touch each other.
The horizontal posture of your arms dictate the height of your chair. The distance from the piano is dictated by the position of your elbows beside your torso.
Almost like you would sit when you were meditating. When you feel that your body, arms and legs are totally relaxed - and of course your neck and head - start playing.
If anything feels odd - start all over by letting your hands hang down to the ground ;-)
Most of the times it is the posture of your fingers and some blocking of your wrists that make you perform corrections of your torso. You start a whole chain of bad behavior of your body.
Start with your fingers (they are the lead actors of your act) - flat horizontal wrist and underarms - distance of your elbows from your torso - distance from the keyboard.
The actually playing of Chopin is a piece of cake.
I as well disagree with the notion of limiting playing to only the fingers.Its almost analogous to attempting to use your pinky without the ring finger moving. I am not a doctor, however,if one moves the fingers, the muscles, tendons, etc,in the forearm move as well. So many other factors are involved with the keyboard other than purely the fingers. One must compensate always when playing.
I recall my teacher having the skeleton of the entire arm hanging on the wall by the piano. I think there is also a basic problem that is involved, that is, the notion of playing 'perfectly'. The obsession with getting every note correct, I am assuming thats the premise of fingers only, is dangerous and can lead to real problems. At least from my experience.
Attempting to perform limiting yourself to fingers, may lead to a 'note perfect' result. Perhaps some professionals
use this method, and succeed, however technically perfect does not make good music. Definitely concentrate on your posture, head to toes. Its ok to make 'mistakes'. Horowitz and Rubenstein made them, so can you. Main thing, if it hurts, its wrong.